The present invention relates to a novel candoluminescent material, which can more particularly be used as a mantle for gas lamps, as well as to the process for producing the same.
Since the work carried out by Auer von Welsbach gas lamp mantles have been formed from a combustible fine fabric impregnates with a mineral which, after the first combustion in the flame of the gas, forms a solid state net giving an intense candoluminescence phenomenon in the flame. The solid net forms rapidly as from the first ignition and acquires its definitive shape and a solid texture, which resists mechanical and thermal shocks. In order to obtain this strength, it is preferable for the solid net to have a fluorine-type cubic crystalline structure. Furthermore, to ensure that the candoluminescent material has a high brightness level in the visible range, it is necessary that it emits little infrared, which makes it possible to reach a high temperature in the virtually colourless flame of the gas.
Previously, the mantles have been formed by a fabric net impregnated with a mixture of salts which, following the first combustion, form a fine and divided fabric of thorium oxide containing a little cerium oxide or other oxides, e.g. an oxide containing 99.2 molar % of thorium oxide and 0.8 molar % of cerium oxide.
However, there are a certain number of disadvantages in the use and production of thorium oxide-base mantles. Thus, thorium is a naturally alpha emitting radioactive element having a half-life of 1.4.multidot.10.sup.10 years and by relationship gives various radioactive isotopes, which are short-life gamma, beta or alpha emitters, including a radioactive gas thoron 220 leading to lead 208. Thus, one tonne of natural thorium represents approximately one curie of .sup.232 Th and one curie of .sup.228 Th. However, the thorium quantities used for the production of mantles are by no means negligible because, according to world statistics, the production of candoluminescent mantles is approximately 300 million per annum. Thus, at a rate of 0.3 g of thorium per mantle, the thorium quantity involved in 100 tonnes per annum, which leads to a dangerous and regrettable dissemination of thorium.
To obviate this disadvantage, recently novel candoluminescent materials have been proposed, which are based on mixtures of zirconium and calcium oxides, cf French Patent application 8,123,202 of Dec. 11, 1981 in the name of the Applicant and entitled "Candoluminescent material, its preparation process and its use as a gas mantle".
A typical composition of such materials described in this French Patent is in accordance with the following molar composition:
75 to 90% of zirconium oxide, PA1 10 to 25% of calcium oxide, PA1 0 to 5% of aluminum oxide and/or magnesium oxide, PA1 0 to 1% in all of iron, manganese, praseodymium and/or cerium oxides.
These compositions are solid solutions having good mechanical properties with regards to their stability in the flame, their resistance to mechanical and thermal shocks and which can irradiate an appropriate luminous energy because, measured in incandescence temperature with the optical pyrometer, it is of the same level as that supplied by the prior art thoria-based compositions.
However, on measuring said luminous energy of the thus obtained mantles according to the lighting technology method, i.e. using a light meter, the figures obtained are below those of commercially available thoria-based mantles.
Mantles formed from zirconium oxide and oxides or rare earths are also known. Thus, German Pat. No. 39,162 describes a candoluminescent material containing approximately 64 molar % of zirconium oxide and approximately 35 molar % of oxides of rare earths, constituted by yttrium and lanthanum.
These high contents of rare earths lower the incandescence temperature and lead to a low luminosity. Thus, in example 2 of this German Patent, the luminosity is below 10 lux.
German Pat. No. 41,945 discloses candoluminescent materials with a molar composition of 25.7% cerium oxide, 16.9% lanthanum oxide, 12.2% yttrium oxide and 45.2% zirconium oxide.
Thus, these mantles contain approximately 55 molar % of rare earths (cerium, lanthanum, yttrium) and have a reddish luminosity well below 5 lux.
Thus, the luminosity of such mantles is inadequate.
The Applicant has therefore continued research in order to attempt to improve the luminosity of zirconium mantles, by defining said luminosity as the magnitude measured by an optical cell, whose sensitivity is representative of that of the human eye.
Thus, among the possible means for improving the light emmission of the prior art candoluminescent refractory compounds, the Applicant investigated novel addition elements based on cubic zirconia, which formed the essential constituent nucleus of the previously developed materials. In this connection, the Applicant's attention was drawn to yttrium, which is known as a cubic zirconia stabilizer and is frequently used for this purpose in molecular proportions roughly the same as those of the solid zirconia/calcium oxide solution. However, mantles obtained on the basis of zirconium/yttrium solutions, although having excellent mechanical properties, have an incandescent radiation in the visible spectrum which is below that of lime-stabilized zirconia mantles. For comparison and all things being equal, it was found that this incandescent radiation was on average 10 lux in the case of a zirconia/yttrium oxide solution. This would appear to make the Expert abandon the use of yttrium for producing candoluminescent solid solutions.
However, in the prior art compositions forming the subject matter of French specification 8,123,202 there are a certain number of magnesium compositions in oxide form having a good luminous energy efficiency, but suffering from the major disadvantage of an embrittlement of the mantle when the magnesia content of the solid solution exceeded 4 to 5% of the molar composition. Thus, for purely mechanical reasons, it would appear to be impossible to further increase the candoluminescent efficiency of compositions obtained by introducing more magnesia into the solid zirconia solution.
However, the Applicant has unexpectedly found that on gradually replacing part of the lime in the prior art compositions by an yttrium oxide fraction, whilst introducing ever increasing quantities of magnesia, the luminosity of the compound was increased, without decreasing its strength, stability and mechanical properties.
In other words, the Applicant has revealed that the use of yttrium oxide, combined or not combined with lime, in a solid candoluminescent composition, makes it possible to considerably increase the magnesia content, thus increasing the strength and stability of the material as well as its luminosity. Thus, according to the invention, it is possible to introduce up to 15 molar % of magnesia.